1. Field of the Invention
The present invention relates in general to a method of microfabrication using energy beams and relates in particular to an ultra-fine microfabrication method which is applicable to making of quantum effect devices, optical lenses, friction reduction devices and fluid seals.
2. Description of the Related Art
Conventional semiconductor device fabrication has been carried out with the use of photolithography as illustrated in FIG. 121. In such a method, those regions of a substrate which are not to be fabricated are covered with a photomask, and the unprotected regions are exposed to an ultra-violet beam for photographic development, or energized ions in the case of plasma processing. The depth of fabrication is controlled by adjusting the time of material etching.
More detailed explanation of photolithographic method will be given below. In step 1, a photoresist material 302 is applied as a coating on the fabrication surface of a substrate base 301. In step 2, a photomask 303 is placed on the target surface which is irradiated with an ultra-violet beam 304, thereby transferring the device pattern 303a formed on the photomask 303 onto the photoresist 302. In step 3, the device pattern 303a is photographically developed to remove the photoresist 302 from the UV-exposed regions of the device pattern 303a so that the fabrication surface of the substrate base 301 will be exposed.
In step 4, selective etching is performed using ions and radicals in a plasma discharge acting on the exposed surface of the base 301, and finally in step 5, the remaining photoresist 302 is removed. By going through the five steps outlined above, the cavities 1c which are identical to the device patterns 303a are formed on the base 301. This basic cycle is repeated to complete the formation of device cavities.
The conventional photolithographic fabrication method is capable of forming cavities having a relatively simple cross sectional profile; however, curvatures and inclined depth profile shapes can only be made by preparing a series of patterns having gradually changing patterns. Fabrication is performed by successively exchanging the patterns and repeating the exposure and development processes to form the curves and complex profiles in stages. This approach is not only time consuming and laborious, but also the precision of the final product is not suitable for microfabrication of advanced devices such as quantum effect devices.
The basic process of photomasking is inherently a complex process involving the steps of: application of photoresist coating, washing, exposure, baking and photographic development. The exposed surface must then be processed by some energy beam to remove the base material, after which the masking must be removed. The overall process is cumbersome and laborious and results in high cost of production. Furthermore, surface roughness and flatness of the fabrication surface affect the precision of pattern making, and thus severely lowering the yield of the process.
Further, the residual photoresist masking material, after the completion of photolithographic processing step, must be removed somehow, and if ashing is used, for example, the quality of surface may be damaged, and if solution is used, contamination or obscurity of shape may result, both of which adversely affect the post-fabrication surface of the product.
The use of plasma for fabrication processing presents a problem of random incident beam angles of ionic particles, and the variation in the incident beam angle is further aggravated by the local charge accumulation in a small surface area. These problems result in a prominent tendency for homogeneous etching, particularly in the case of micro-fabrication processing, and produces devices with low flatness at the bottom of etched grooves and low verticality of the side walls of the grooves. These problems present a severe limitation in the precision of fabrication, particularly for making device patterns in the ultra-fine range of less than 1 .mu.m.